Blank space at the end should select remaining items

[ghost]

When the items don't fill the last row of the grid, there's a blank space. If you drag until that space, the remaining items should be selected.

Check this out:

This behavior is arguably right, so I think this is more a missing feature than a bug.

I'll eventually assign myself to this issue if nobody else do. Just notice that this should be reproduced in drag_select_grid_view_test.dart before fixing.